Add working animated ggplot example

[andrewmarx]

Currently, in the "Animations" tutorial, using gifski with base plot produces a working output. The gganimate example technically works, but does not adjust the color scale across frames, resulting in a less useful visual. Unless gganimate adds built in support for dynamic color scales, a working animated ggplot example could be accomplished using gifski and ggplot, as mentioned in the tutorial's introduction.